Optical recording medium and recording-reproducing apparatus

ABSTRACT

Disclosed is an optical recording medium, including a substrate, a reflective film on the substrate, a super resolution film made of an optical material whose complex refractive index is changed in accordance with an intensity of a light irradiating the super resolution film, a first thin film interference section constituted by plural transparent thin films, the plural transparent thin films being stacked one another, each two of the plural transparent thin films adjacent to each other being different from each other in refractive index, and the super resolution film and the first thin film interference section forming a laminate structure between the substrate and the reflective film or on the reflective film, and a recording film optionally provided between the laminate structure and the reflective film. Also disclosed is a recording-reproducing apparatus having such a medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is based upon and claims the benefit of priorityfrom the prior Japanese Patent Applications No. 11-374994, filed Dec.28, 1999; and No. 2000-089615, filed Mar. 28, 2000, the entire contentsof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to an optical recording medium anda recording-reproducing apparatus, particularly, to an optical recordingmedium having a super resolution film and a recording-reproducingapparatus using the particular optical recording medium.

[0003] An optical disk memory capable of reproducing information orcapable of recording and reproducing information by means of light-beamirradiation has excellent characteristics. For example, the optical diskmemory has a large capacity, is capable of a quick-access, and capableof detachably mounting an optical disk. Therefore, the optical diskmemory has already been put to a practical use as a memory device forstoring various data such as voice, image, and computer data and isexpected to become further pervasive.

[0004] As a technique for increasing the recording density of an opticaldisk, it is considered to shorten the wavelength of the gas laser usedfor cutting a master, to increase the numerical aperture of an objectivelens, and to decrease the thickness of the substrate included in theoptical disk. Further, when it comes to an optical disk capable of notonly reproduction but also recording, various approaches are beingstudied including the mark-length recording and the land-grooverecording.

[0005] In addition to the technique for increasing the recording densitydescribed above, also proposed as a technique for effectively increasingthe recording density is a super resolution technology utilizing a superresolution film. The super resolution technology was originally proposedas a technology peculiar to a magneto-optical disk. Then, an attempt toreproduce at a super resolution by providing a super resolution film,whose transmittance is changed by the light irradiation, on the side ofthe light irradiation surface of a ROM disk was reported. In this way,it has now been proved that the super resolution technology can beapplied to all the optical disks including the magneto-optical disk, aCD-ROM, a CD-R, a WORM and a phase change type optical recording medium.

[0006] The super resolution technology can be roughly classified into aheat mode type and a photon mode type. These two types differ from eachother in the material forming the super resolution film.

[0007] For example, in the heat mode type, a material that changes itsphase by heating is used for the super resolution film. If such a superresolution film is irradiated with a laser beam, formed is a temperaturedistribution in which the temperature is lowered from the center of thebeam spot toward the periphery. As a result, an optical opening having ahigher refractive index is formed in the portion heated to a temperaturehigher than the phase transition temperature of the super resolutionfilm. It follows that it is possible to form a very small opticalopening by controlling the temperature distribution of the superresolution film.

[0008] On the other hand, in the photon mode system, a photochromicmaterial that develops color or quenches color upon irradiation withlight is used for forming a super resolution film. If a photochromicmaterial is irradiated with light having an energy higher than apredetermined value, the electron is excited from the ground level to anexcited level having a short life and, then, is transited from theexcited level to a metastable excited level having a very long life. Asa result, the light absorption characteristics are changed. Also, in thephoton mode system, there is an example that a semiconductor continuousfilm or a semiconductor fine particle dispersion film utilizing anabsorption saturation phenomenon is used as the super resolution film.

[0009] In the case of employing any of the heat mode system and thephoton mode system, the characteristics of the super resolution film aredependent on the rate of change in the optical constant (real partand/or imaginary part of the complex refractive index) of the superresolution film relative to the intensity of the irradiating light. Tobe more specific, with increase in the change of the optical constant,it is possible to form a large difference in reflectance between theoptical opening formed within the beam spot and the optical mask portionaround the optical opening, making it possible to realize an excellentreproducing performance.

[0010] However, it is practically difficult to find a material whoseoptical constant is greatly changed by light irradiation. As apparentfrom the above description, in the case of forming the super resolutionfilm with a material whose optical constant is changed only slightly bythe light irradiation, it is impossible to produce a large difference inreflectance between the optical opening and the optical mask portion.Therefore, in such a case, a read error is likely to take place. Underthe circumstances, the material that can be used for forming the superresolution film is much limited in the prior art.

BRIEF SUMMARY OF THE INVENTION

[0011] An object of the present invention is to provide an opticalrecording medium and a recording-reproducing apparatus capable ofachieving a high recording density.

[0012] Another object of the present invention is to provide an opticalrecording medium and a recording-reproducing apparatus that are unlikelyto cause a read error.

[0013] Still another object of the present invention is to provide anoptical recording medium and a recording-reproducing apparatus thatpermit a material whose optical constant is changed only slightly uponirradiation with light to be used for forming the super resolution film.

[0014] According to a first aspect of the present invention, there isprovided an optical recording medium, comprising a substrate, areflective film provided on the substrate and having a recessed portionor a projecting portion as a recording mark on a surface thereof, asuper resolution film made of an optical material whose complexrefractive index is changed in accordance with an intensity of a lightirradiating the super resolution film, and a first thin filminterference section consisting of a plurality of transparent thinfilms, the plural transparent thin films being stacked one another, eachtwo of the plural transparent thin films adjacent to each other beingdifferent from each other in refractive index, and the super resolutionfilm and the first thin film interference section forming a laminatestructure between the substrate and the reflective film or on thereflective film.

[0015] According to a second aspect of the present invention, there isprovided an optical recording medium, comprising a substrate, areflective film provided on the substrate, a super resolution film madeof an optical material whose complex refractive index is changed inaccordance with an intensity of a light irradiating the super resolutionfilm, a first thin film interference section consisting of a pluralityof transparent thin films, the plural transparent thin films beingstacked one another, each two of the plural transparent thin filmsadjacent to each other being different from each other in refractiveindex, and the super resolution film and the first thin filminterference section forming a laminate structure between the substrateand the reflective film or on the reflective film, and a recording filmprovided between the laminate structure and the reflective film.

[0016] According to a third aspect of the present invention, there isprovided a recording-reproducing apparatus, comprising an opticalrecording medium comprising a substrate, a reflective film provided onthe substrate, a super resolution film made of an optical material whosecomplex reflective index is changed in accordance with an intensity of alight irradiating the super resolution film, a first thin filminterference section consisting of a plurality of transparent thinfilms, the plural transparent thin films being stacked one another, eachtwo of the plural transparent thin films adjacent to each other beingdifferent from each other in refractive index, and the super resolutionfilm and the first thin film interference section forming a laminatestructure between the substrate and the reflective film or on thereflective film, and a recording film provided between the laminatestructure and the reflective film, a recording mechanism configured toirradiate the recording film with a recording light so as to formrecording marks corresponding to information to be recorded in therecording film, and a reproducing mechanism configured to irradiate therecording film with light and detect the light reflected from theoptical recording medium so as to reproduce the information recorded asthe recording marks in the recording film.

[0017] The term “refractive index” used without the term “complex”represents the real part of the complex refractive index. Also, the term“reflectance”, which is included in the expressions such as “reflectanceof the optical recording medium” and “reflectance of the optical disk”,represents the value observed in the case where light is emitted to theoptical recording medium or the optical disk from the side of the superresolution film and the thin film interference section toward thereflective film.

[0018] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bymeans of the instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0019] The accompanying drawings, which are incorporated in andconstitute a part of the specification, illustrate presently preferredembodiments of the invention, and together with the general descriptiongiven above and the detailed description of the preferred embodimentsgiven below, serve to explain the principles of the invention.

[0020]FIG. 1 is a cross sectional view schematically showing an opticalrecording medium according to one embodiment of the present invention;

[0021]FIG. 2 is a graph showing the change in the transmittance of thelaminate structure formed by the thin film interference section and thesuper resolution film included in the optical recording medium shown inFIG. 1;

[0022]FIGS. 3A and 3B are views schematically showing the beam diameterdiminishing effect achieved by the change in the transmittance shown inFIG. 2;

[0023]FIG. 4 is a view schematically showing a recording-reproducingapparatus having the optical recording medium shown in FIG. 1;

[0024]FIG. 5 is a cross sectional view schematically showing an opticalrecording medium according to Example 1 of the present invention;

[0025]FIG. 6 is a graph showing the relationship between theconstruction of the thin film interference section and the reflectanceof the optical recording medium shown in FIG. 5;

[0026]FIG. 7 is a graph showing the relationship between the refractiveindex of the super resolution film and the reflectance of the opticalrecording medium shown in FIG. 5;

[0027]FIG. 8 is a graph showing the relationship between the pit lengthand the CNR value in respect of the optical recording medium accordingto Example 2 of the present invention and the optical recording mediumfor the comparative case;

[0028]FIG. 9 is a cross sectional view schematically showing the opticalrecording medium according to Example 4 of the present invention;

[0029]FIG. 10 is a graph showing the relationship between the distancebetween adjacent recording marks and the CNR value in respect of theoptical recording medium according to Example 4 of the present inventionand the optical recording medium for the comparative case;

[0030]FIG. 11 is a graph showing the effect of the thin filminterference section on the reflectance of the optical recording mediumaccording to Example 5 of the present invention;

[0031]FIG. 12 is a graph showing the relationship between the refractiveindex of the transparent thin film and the reflectance of the opticalrecording medium according to Example 6 of the present invention;

[0032]FIG. 13 is a graph showing the relationship between ΔR, which isthe difference in reflectance between when the light having a lowintensity is irradiated and when the light having a high intensity isirradiated, and Δn, which is the difference in refractive index betweenadjacent transparent thin films, in respect of the optical recordingmedium according to Example 6 of the present invention; and

[0033]FIG. 14 is a graph showing the relationship between the intensityof the irradiating light and the reflectance in respect of the opticalrecording medium according to Example 7 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0034] The present invention will now be described more in detail withreference to the accompanying drawings. Throughout the drawings, thesame or similar constituents are denoted by the same reference numeralsso as to avoid an overlapping description.

[0035]FIG. 1 is a cross sectional view schematically showing an opticalrecording medium 1 according to one embodiment of the present invention.The optical recording medium 1 shown in FIG. 1 has a transparentsubstrate 2 and a counter substrate 7. Arranged between thesetransparent substrate 2 and the counter substrate 7 are a thin filminterference section 3 as a first thin film interference section, asuper resolution film 4, a recording film 5 and a reflective film 6,which are superposed one upon the other in the order mentioned from theside of the transparent substrate 2. In the case of reading theinformation recorded on the optical recording medium 1 or in the case ofrecording information on the optical recording medium 1, the opticalrecording medium 1 is irradiated with a light-beam emitted from the sideof the transparent substrate 2 toward the recording film 5. It should benoted that the thin film interference section 3 has transparent thinfilms 3A and 3B differing from each other in the refractive index andarranged to form a laminate structure.

[0036] Before describing in detail the construction of the opticalrecording medium 1 shown in FIG. 1, the principle utilized in theoptical recording medium 1 will now be described with reference to FIGS.2, 3A and 3B.

[0037] Specifically, FIG. 2 is a graph showing the change in thetransmittance of the laminate structure formed by the thin filminterference section 3 and the super resolution film 4 shown in FIG. 1.In the graph, the intensity of the light-beam irradiating the laminatestructure is plotted on the abscissa, with the transmittance beingplotted on the ordinate. Curve 11 in the graph represents data obtainedin conjunction with the laminate structure formed by the thin filminterference section 3 and the super resolution film 4. On the otherhand, curve 12 represents the data obtained in the case where the superresolution film 4 alone is provided without providing the thin filminterference section 3.

[0038] Where the thin film interference section 3 is not provided, thechange in the transmittance relative to the change in the intensity ofthe light-beam is very small as apparent from curve 12 shown in FIG. 2.On the other hand, where the thin film interference section 3 isprovided, multiple reflection and multiple interference take place inthe thin film interference section 3. Therefore, in the case ofproviding the thin film interference section 3, by setting appropriatelythe optical characteristics of the thin film interference section 3, itis possible to increase markedly the change in the transmittancerelative to the change in the intensity of the light-beam as comparedwith the case where the thin film interference section 3 is not providedas apparent from curve 11. In other words, it is possible to amplify thecharacteristics of the super resolution film 4 to change thetransmittance by providing the thin film interference section 3.

[0039]FIGS. 3A and 3B schematically show the beam diameter diminishingeffect achieved by the change in the transmittance shown in FIG. 2. Ingeneral, a light-beam such as a laser beam has an intensity profile,which resembles the Gaussian distribution in which the temperature islowered from the central portion toward the periphery, as denoted bycurve 13 in FIG. 3A.

[0040] Where such a light-beam is allowed to be incident on the superresolution film 4 alone, the intensity profile 15 of the transmittedlight is scarcely changed from the intensity profile 13 of theirradiating light as shown in FIGS. 3A and 3B because the change in thetransmittance of the super resolution film 4 is small as denoted by thecurve 12. In other words, a so-called “super resolution effect”, whichis an effect of diminishing the beam diameter, is very small in thiscase.

[0041] On the other hand, where the light-beam having the intensityprofile 13 shown in FIG. 3A is allowed to be incident on the laminatestructure of the thin film interference section 3 and the superresolution film 4, the intensity profile 14 of the transmitted light isrendered much steeper than the intensity profile 13 of the irradiatinglight as shown in FIG. 3B, because the laminate structure noted aboveexhibits a large change in the transmittance as denoted by the curve 11shown in FIG. 2. In other words, it is possible to obtain a very largesuper resolution effect, in this case.

[0042] As described above, a very large super resolution effect can beobtained in the case of providing the thin film interference section 3.It follows that the optical recording medium shown in FIG. 1 permitsrealizing a very high recording density as compared with the prior art.It should also be noted that, where the optical recording medium shownin FIG. 1 is made substantially equal to the conventional recordingmedium in the recording density, problems such as crosstalk is unlikelyto take place. In other words, a failure to read informationsatisfactorily is unlikely to take place. It follows that it is possibleto widen the operating margin of the recording-reproducing apparatus andthe reproducing apparatus. Further, since the presence of the thin filminterference section makes it possible to markedly increase the superresolution effect, it is possible for the super resolution film 4 to beformed of a material whose optical constant is slightly changed by thelight irradiation.

[0043] The material, etc. used for preparing the members of the opticalrecording medium 1 shown in FIG. 1 will now be described.

[0044] Specifically, it is possible for each of the substrates 2 and 7included in the optical recording medium 1 shown in FIG. 1 to be a resinsubstrate made of, for example, polycarbonate, polymethyl methacrylateand polyolefin, which is generally used in the optical disk, a substratehaving a photopolymer layer formed on a glass substrate, or atransparent substrate such as a glass substrate. Incidentally, any oneof the substrates 2 and 7 is an optional constituent in this embodiment.Also, a transparent substrate is used as the substrate 2 on the side ofthe light irradiation. However, the substrate 7 positioned on the sideopposite to the light irradiation side does not necessarily require tobe transparent.

[0045] It is desirable for the facing surface of at least one of thesesubstrates 2 and 7 to have a tracking groove formed by a masteringprocess. Also, where the optical recording medium 1 does not include therecording film 5, i.e., where the medium 1 is read-only type, it isdesirable for at least one facing surface of the substrates 2 and 7 tohave pits or a tracking groove formed by a mastering process.

[0046] In the optical recording medium 1 shown in FIG. 1, it isdesirable for the transparent thin films 3A and 3B collectively formingthe thin film interference section 3 to be formed of a transparentmaterial having an extinction coefficient of substantially zero, thoughit is possible to use a material having a small extinction coefficientin the case where the effect described above can be obtained. It ispossible for each of the transparent thin films 3A and 3B to be formedof, for example, an oxide such as SiO₂, Al₂O₃, ZrO₂, and TiO₂; afluoride such as MgF₂ and CaF₂; a nitride such as AlN and Si₃N₄; asulfide such as ZnS; and a mixture thereof. For example, it is possiblefor one of the transparent thin films 3A and 3B, which has a higherrefractive index, to be formed of ZrO₂, TiO₂, ZnS or ZnS.SiO₂, and forthe other, which has a lower refractive index, to be formed of MgF₂,CaF₂, SiO₂, Al₂O₃, or Na₃Al₂F₆. It is also possible to use a thin filmof Au, GeSi, or GeSn as each of the transparent thin films 3A and 3B ifthe film of Au, etc. is sufficiently thin. Further, it is possible touse a polymer film of C—H series or C—F series as each of thetransparent thin films 3A and 3B.

[0047] In the optical recording medium 1 shown in FIG. 1, it is possiblefor the transparent thin film 3A to have a higher refractive index andfor the transparent thin film 3B to have a lower refractive index.Alternatively, it is possible for the transparent thin film 3A to have alower refractive index and for the transparent thin film 3B to have ahigher refractive index. What should be noted is that it suffices forthe adjacent transparent thin films 3A and 3B to be different from eachother in the refractive index. Also, in the optical recording medium 1shown in FIG. 1, the thin film interference section 3 consists of thetransparent thin films 3A and 3B alone. However, it is possible for thethin film interference section 3 to be formed of three or moretransparent thin films. In this case, it is possible to laminatealternately transparent thin films having a higher refractive index andtransparent thin films having a lower refractive index such that a filmhaving a higher refractive index is positioned adjacent to a film havinga lower refractive index. It is also possible to laminate transparentthin films such that the refractive index is decreased or increased fromthe substrate 2 toward the substrate 7.

[0048] It is desirable for each of the transparent thin films 3A and 3Bto meet the relationship represented by the equation given below:

d _(i)=λ/4n _(i) +m+λ/n _(i), and

[0049] it is more desirable for each of the transparent thin films 3Aand 3B to meet the relationship given below unless there is a specialreason:

d _(i)=λ/4n _(i)

[0050] where d_(i) represents the thickness of each of the transparentthin films 3A and 3B, n_(i) denotes the refractive index of each of thetransparent thin films 3A and 3B, λ denotes the wavelength of theirradiating light, and m denotes 0 or a natural number.

[0051] Where each of the transparent thin films 3A and 3B meets therelationship specified by the equation or equations given above, it ispossible to enable the thin film interference section 3 to produce themaximum optical interference effect. Also, if the difference between theleft term and the right term in each of the equations given above iswithin the range of about ±20%, a sufficient effect can be produced.

[0052] In the optical recording medium 1, the super resolution film 4may be the one which changes mainly the refractive index (real part ofthe complex refractive index) in accordance with the intensity of theirradiating light. Alternatively, the super resolution film may be theone which changes mainly the extinction coefficient (imaginary part ofthe complex refractive index) in accordance with the intensity of theirradiating light. In other words, the real part n of the complexrefractive index of the optical material constituting the superresolution film 4 can have a rate of change relative to the intensity ofthe irradiating light larger than or smaller than that of the imaginarypart k of the complex refractive index of the optical material. Itshould be noted in this connection that the optimum value of the opticaldesign differs depending on the factor as to which of the refractiveindex n or the extinction coefficient k is mainly changed.

[0053] Where the optical material constituting the super resolution film4 is the one which changes mainly the refractive index n in accordancewith the intensity of the irradiating light, it is desirable for theoptical recording medium 1 to meet the relationship specified by theinequality given below:

|n ₂ −n ₁ |/n ₁≦150×|R ₂ −R ₁ |/d

[0054] where n₁ denotes the refractive index of the optical materialunder the irradiation of light having a first intensity, n₂ denotes therefractive index of the optical material under the irradiation of lighthaving a second intensity higher than the first intensity, d denotes thethickness (nm) of the super resolution film 4, R₁ denotes thereflectance of the optical recording medium 1 when the optical materialhas a refractive index n₁, and R₂ denotes the reflectance of the opticalrecording medium 1 when the optical material has a refractive index n₂.

[0055] Where the optical recording medium 1 meets the relationshipspecified in the inequality given above, it is reasonable to state thatthe optical recording medium 1 effectively utilizes the effect of thepresent invention.

[0056] Where the super resolution film 4 of the optical recording medium1 is of the heat mode type, it is possible for the super resolution film4 to be formed of a phase change material such as Ge—Sb—Te, Sb—Te andSb, or a thermochromic material such as spiropyran. On the other hand,where the super resolution film 4 of the optical recording medium 1 isof the photon mode type, it is possible for the super resolution film 4to be formed of a photochromic material such as pyrobenzopyran, fulgide,diarylethene, cyclophane, and azobenzene and an absorption saturationseries material such as a semiconductor film and a semiconductor fineparticle dispersion film.

[0057] It is possible to select appropriately the material of thesemiconductor film or the semiconductor particles contained in thesemiconductor particle dispersion film in accordance with the wavelengthof the laser beam used. For example, it is possible to use a halide ofCu or Ag, a Cu oxide, AgSe, AgTe, SrTe, SrSe, CaSi, ZnS, ZnO, ZnSe,ZnTe, CdS, CdSe, CdTe, AlTe, InS, InO, InSe, InTe, AlSb, AlN, AlAs, GaN,GaP, GaAs, GaSb, GeS, GeSe, SnS, SnSe, SnTe, PbO, SiC, AsTe, AsSe, SbS,SbSe, SbTe, BiS, TiO, MnSe, MnTe, FeS, MoS, CuAlS, CuInS, CuInSe,CuInTe, AgInS, AgInSe, AgInTe, ZnSiAs, ZnGeP, CuSbS, CuAsS, AgSbS andAgAs. Also, in the case of using a semiconductor fine particledispersion film, it is possible for the semiconductor fine particles tobe dispersed in, for example, a transparent dielectric material such asSiO₂, Si₃N₄, Ta₂O₅, TiO₂ and ZnS.SiO₂, a plasma polymerization materialsuch as C—H series and a C—F series, and C.

[0058] The optical recording medium 1 shown in FIG. 1 has the recordingfilm 5 and, thus, is capable of both reproduction and recording. It ispossible for the recording film 5 to be of the type that erase and writeof information can be performed repeatedly by utilizing light such as amagneto-optical recording film of a magneto-optical recording medium ora phase change recording film of a phase change recording medium. It isalso possible for the recording film 5 to be of the type thatinformation can be written only once like a recording film using a dye.It should be noted that the recording film 5 is an optional constituentin the optical recording medium 1 shown in FIG. 1. In other words, wherethe optical recording medium 1 is of the read-only type, the recordingfilm 5 is not necessary in the optical recording medium 1.

[0059] In the optical recording medium 1 shown in FIG. 1, it is possiblefor the reflective film 6 to be formed of, for example, a metal having arelatively high reflectance such as Al, Au, Cu and Ag, and an alloyprepared by adding Ti, Mo, Pd or Cr to the metal noted above.

[0060] In the optical recording medium shown in FIG. 1, pits as arecording marks and/or a tracking groove are formed on one main surfaceof the substrate 2 by the mastering process and, then, the thin filminterference section 3, the super resolution film 4, the recording film5 and the reflection 6 are formed successively on the main surface ofthe substrate 2 in most cases. Alternatively, it is also possible toform pits and/or a tracking groove on one main surface of the substrate7, followed by forming successively the reflection 6, the recording film5, the super resolution film 4 and the thin film interference section 3on the main surface of the substrate 7 in the order mentioned.

[0061] In the optical recording medium 1, the order of laminating thethin film interference section 3, the super resolution film 4 and therecording film 5 is not particularly limited, though it is desirable tolaminate the thin film interference section 3, the super resolution film4 and the recording medium 5 in the order mentioned starting with thelight irradiation side. It is also possible to laminate the superresolution film 4, the thin film interference section 3 and therecording film 5 in the order mentioned starting with the lightirradiation side. Further, in the optical recording medium 1, it ispossible to form a protective film on each surface of the recording film5 in order to prevent the evaporation thereof and the like.

[0062] Where the optical recording medium 1 does not include therecording film 5, i.e., where the medium 1 is of the read-only type, itis desirable to arrange a second thin film interference section betweenthe super resolution film 4 and the reflective film 6. The presence ofthe second thin film interference section makes it possible to achieve afurther optimization of the optical response.

[0063] On the other hand, where the optical recording medium 1 includesthe recording film 5, it is desirable to arrange a second thin filminterference section between the super resolution film 4 and therecording film 5 or to arrange the third thin film interference sectionbetween the recording film 5 and the reflective film 6. Also, it is moredesirable to arrange both second and third interference sections. Afurther optimization of the optical response can be achieved in thiscase, too.

[0064] It is possible for each of the second and third thin filminterference sections to be formed of a single transparent thin film ora laminate structure of a plurality of transparent thin films. Whereeach of the second and third thin film interference sections is formedof a laminate structure of a plurality of transparent thin films, it ispossible to allow the adjacent transparent thin films to be differentfrom each other in the refractive index or in the characteristics otherthan the optical characteristics such as the hardness or weatherresistance.

[0065] A recording-reproducing apparatus using the optical recordingmedium 1 described above will now be described.

[0066] Specifically, FIG. 4 schematically shows a recording-reproducingapparatus 21 including the optical recording medium 1 shown in FIG. 1.As shown in the drawing, the recording-reproducing apparatus 21 has theoptical recording medium 1, a spindle motor 22, an optical head 23, anarm 24, a linear motor 25, an interface 26, a drive controller 27, adrive control circuit 28, a modulation circuit 29, a laser driver 30, apickup 31, a preamplifier 32, a variable gain amplifier 33, an A/Dconversion circuit 34, a linear equalization circuit 35, a datadetection circuit 36 and a decoder 37.

[0067] In the recording-reproducing apparatus 21 shown in FIG. 4, theoptical recording medium 1 is in the form of an optical disk of arewritable type, e.g., a phase change type, and is rotatably supportedby a rotating shaft of the spindle motor 22 such that the substrate 2 ofthe optical disk 1 faces upward. The optical disk 1 is allowed to berotated at a predetermined rotational speed by controlling therotational speed of the spindle motor 22. The optical head 23 supportedby one end of the arm 24 is arranged above the optical disk 1. Thelinear motor 25 is mounted to the other end of the arm 24 to permit theoptical head 23 to be movable in the radial direction of the opticaldisk 1. These spindle motor 22 and the linear motor 25 are controlled bythe drive controller 27 via the drive control circuit 28. The opticaldisk 1 is made movable relative to the optical head 23 by the drivingmechanism of the particular construction described above.

[0068] In the recording-reproducing apparatus 21 shown in FIG. 4, theoptical head 23, the modulation circuit 29, the laser driver 30 and thepickup 31 constitute a recording mechanism. The optical head 23 includesa light source such as a laser diode, and permits the optical disk 1 tobe irradiated with the laser light emitted as a recording light from thelaser diode. Also, the optical head 23 receives the light reflected fromthe optical disk 1 and guides the received light to a detection element.The modulation circuit 29 executes the coding process for converting therecording data transmitted from the drive controller 27 into apredetermined sign bit string. Further, the laser driver 30 drives thelaser diode arranged within the pickup 31 so as to permit recordingmarks corresponding to the sign bit string outputted from the modulationcircuit 29 to be formed in the recording film 5 of the optical disk 1.

[0069] In the recording-reproducing apparatus 21 shown in FIG. 4, thelight detection system including the optical head 23 and the pickup 31and a reproduced signal conditioning circuit constitute a reproducingmechanism. Incidentally, the reproduced signal conditioning circuitincludes the preamplifier 32, the variable gain amplifier 33, the A/Dconversion circuit 34, the linear equalization circuit 35, the datadetection circuit 36 and the decoder 37. In addition to the laser diode,a detection element is also arranged within the pickup 31. In readingthe information recorded in the recording film 5 of the optical disk 1,the optical disk 1 is irradiated via the optical head 23 with the laserlight emitted from the laser diode of the pickup 31. The light reflectedfrom the optical disk 1 is guided through the optical head 23 to thepickup 31. A detection element including a light detector is arranged inthe pickup 31. The intensity of the reflected light or the reflectance,which is a ratio of the intensity of the reflected light to theintensity of the irradiating light, is detected by the detectingelement.

[0070] The preamplifier 32 and the variable gain amplifier 33 serve toamplify the output signal from the detection element of the pickup 31.The A/D converter 34 serves to convert the signal amplified by thepreamplifier 32 and the variable gain amplifier 33 into a digitalsignal. The linear equalization circuit 35 is a kind of a digitalfilter. The data detection circuit 36 is for example a signal processingcircuit which estimates the sign bit string by a maximum likelihood (ML)method for detecting data from the waveform of the reproduced signalequalized by the partial response (PR). Further, the decoder 37 servesto bring the sign bit string detected by the data detection circuit 36back to the original recording data.

[0071] The driving controller 27, which is a main control system of therecording-reproducing system shown in FIG. 4, is connected to, forexample, a personal computer or an AV appliance via the interface 26 soas to control the transmission of the recording and reproducing data.

[0072] The recording-reproducing apparatus 21 has the optical disk 1shown in FIG. 1, as already described. Therefore, therecording-reproducing apparatus 21 permits achieving a recording densityhigher than that in the prior art. Also, where the recording density ofthe recording-reproducing apparatus 21 is made substantially equal tothat of the conventional apparatus, a difficulty such as crosstalk isunlikely to be produced. In other words, a read error is unlikely totake place. It follows that the operating margin can be widened.

[0073] Some Examples of the present invention will now be described.

EXAMPLE 1

[0074]FIG. 5 is a cross sectional view schematically showing an opticalrecording medium 1 according to Example 1 of the present invention. Theoptical recording medium 1 shown in FIG. 5 is a ROM disk having pitsformed as recording marks on one main surface of the transparentsubstrate 2 made of polycarbonate. The thin film interference section 3,the super resolution film 4 and the reflective film 6 are successivelyformed in the order mentioned on that surface of the transparentsubstrate 2 on which the pits are formed. Recessed portionscorresponding to the pits formed on the surface of the substrate 2 areformed on each of these thin film interference section 3, superresolution film 4 and reflective film 6.

[0075] In the ROM disk 1, the interference light generated by theinterference between the light reflected from the reflective film 6 andthe light subjected to a multiple interference in the thin filminterference section 3 is utilized as a reproducing light in the step ofthe light irradiation from the side of the transparent substrate 2 forreading information. In Example 1, the presence of the thin filminterference section 3 makes it possible to increase a ratio of asignal, which represents the intensity of the reflected light from theoptical opening, to a noise, which represents the intensity of thereflected light from the optical mask portion, i.e., an S/N ratio. Theterm “optical opening” noted above represents the portion where thelight intensity has a value not lower than a predetermined value incurve 14 shown in FIG. 3B. On the other hand, the term “optical maskportion” noted above represents the portion where the light intensity islower than the predetermined value in curve 14 shown in FIG. 3B.

[0076] Also, in the ordinary ROM disk, a reflective film is formeddirectly on a substrate, and information is read by irradiating thereflective film with light from the side opposite to the side of thesubstrate. In the ROM disk 1 of the present invention shown in FIG. 5,however, information is read by irradiating the reflective film 6 withlight from the side of the substrate 2. As a result, the diameter of theincident light-beam is diminished by the thin film interference section3 and the super resolution film 4 and, then, the incident light isreflected from the interface between the super resolution film 4 and thereflective film so as to be brought back to the detecting system. Inother words, in the ROM disk 1 shown in FIG. 5, the informationtransferred from the surface of the transparent substrate 2 to thereflective film 6 is read in place of the information recorded on thesurface of the transparent substrate 2.

[0077] The situations common with the construction employed in Example 1will now be described in detail.

[0078] Specifically, in the ROM disk 1 of Example 1, the thin filminterference section 3 has a laminate structure comprising a transparentthin films 3A, each of which is made of ZnS and has a refractive indexof 2.35, and transparent thin films 3B, each of which is made of MgF₂and has a refractive index of 1.4. These transparent thin films 3A and3B are alternately stacked upon the other. In the ROM disk 1 shown inFIG. 5, three transparent thin films are stacked one upon the other.Incidentally, the refractive index noted above is based on the lighthaving a wavelength λ of 410 nm. It is desirable for the thickness ofeach of the transparent thin films 3A and 3B to be λ/4n or in thevicinity of λ/4n, where n represents the refractive index of each ofthese transparent thin films. In Example 1, the thickness of thetransparent thin film 3A is set at about 44 nm, and the thickness of thetransparent thin film 3B is set at about 70 nm.

[0079] The super resolution film 4 is formed of a material having arefractive index of 1.7 and an extinction coefficient of substantiallyzero, i.e., less than 0.1, when the material is irradiated with lighthaving a low intensity. The thickness of the super resolution film 4 isset at about 87 nm so as to minimize the reflectance of the ROM disk 1when the super resolution film 4 has the refractive index noted above.Incidentally, the term “light having a low intensity” noted aboverepresents the light component outside the FWHM (Full Width at HalfMaxima) of the light (about 1 mW) used for reproducing the generaloptical disk. On the other hand, the term “light having a highintensity” used as a contrast to the “light having a low intensity”noted above represents the light component inside the FWHM of the light(about 1 mW) used for reproducing the general optical disk. It should benoted in this connection that it suffices to determine appropriately thepower of the light used for reproducing the optical disk 1 in accordancewith the power response characteristics of the super resolution film 4.In general, it is possible to set the power of the light to fall withina wide range of between about 0.3 mW and about 5 mW.

[0080] As the super resolution film 4 having the above-noted values ofthe complex refractive index when irradiated with a light having a lowintensity, a film prepared by dispersing fine particles of a phasechange material such as Ge—Sb—Te, Sb—Te and Sb in a transparentdielectric material so as to control the complex refractive index; afilm prepared by dispersing a thermochromic material such as bianthroneand spiropyran in a solvent so as to control the complex refractiveindex; a film prepared by dispersing a photochromic material such aspyrobenzopyran, fulgide, diarylethene, cyclophane and azobenzene in asolvent so as to control the complex refractive index; a film havingsemiconductor fine particles dispersed therein and the like can be used.Such a super resolution film 4 can be prepared by, for example, amulti-source simultaneous sputtering method, a spin coating method or aco-vapor deposition method.

[0081] The reflection spectra were calculated in respect of a pluralityof ROM disks 1 of the construction described above, the ROM disks 1differing from each other in the construction of the thin filminterference section 3. FIG. 6 shows the results.

[0082] Specifically, FIG. 6 is a graph showing the relationship betweenthe construction of the thin film interference section 3 and thereflectance of the ROM disk 1. In the graph of FIG. 6, the wavelength ofthe light irradiating the ROM disk 1 is plotted on the abscissa, withthe reflectance at the optical mask portion in the step of the lightirradiation being plotted on the ordinate. Curves 41, 42, 43 and 44 areshown in the graph of FIG. 6. Curve 41 represents the data in the casewhere the thin film interference section 3 was formed of a singletransparent thin film 3A alone (H). Curve 42 represents the data in thecase where the thin film interference section 3 was prepared by stackinga single transparent thin film 3A and a single transparent thin film 3Bone upon the other from the substrate 2 (HL). Curve 43 represents thedata in the case where the thin film interference section 3 was preparedby allowing a single transparent thin film 3B to be sandwiched betweentwo transparent thin films 3A (HLH). Further, curve 44 represents thedata in the case where the thin film interference section 3 was preparedby alternately stacking three transparent thin films 3A and twotransparent thin films 3A one upon the other (HLHLH).

[0083] As apparent from curve 41 shown in FIG. 6, where the thin filminterference section 3 is formed of a single transparent thin film 3Aalone, the reflectance in relation to the light (wavelength of 410 nm)used for reading information is substantially equal to the reflectancein relation to the light of the other wavelength region. On the otherhand, where the thin film interference section 3 is formed of at leastone transparent thin film 3A and at least one transparent thin film 3B,the reflectance in relation to the light having a wavelength of 410 nmis markedly lower than the reflectance in relation to the light havingother wavelength region, as apparent from curves 42 to 44. This tendencyis rendered prominent with increase in the number of transparent thinfilms 3A and 3B forming the thin film interference section 3.Incidentally, the reflectance was calculated by the known calculatingmethod.

[0084] Then, the relationship between the refractive index and thereflectance of the super resolution film 4 was examined in respect ofthe ROM disk 1 shown in FIG. 5, in which the thin film interferencesection 3 was prepared by alternately laminating three transparent thinfilms 3A and two transparent thin films 3B (HLHLH).

[0085]FIG. 7 is a graph showing the relationship between the refractiveindex of the super resolution film 4 and the reflectance of the ROM disk1. In the graph of FIG. 7, the wavelength of the light irradiating theROM disk 1 is plotted on the abscissa, with the reflectance in the caseof irradiating the light being plotted on the ordinate. Curves 51, 52,53 and 54 are shown in the graph of FIG. 7. Curve 51 represents the datein the case where the refractive index of the super resolution film 4was 1.7. Curve 52 represents the date in the case where the refractiveindex of the super resolution film 4 was 1.71. Curve 53 represents thedate in the case where the refractive index of the super resolution film4 was 1.75. Further, curve 54 represents the date in the case where therefractive index of the super resolution film 4 was 1.8. Incidentally,the super resolution film 4 was assumed such that, with increase in theintensity of the irradiating light, the real part of the complexrefractive index would be increased, and the imaginary part of thecomplex refractive index would remain substantially unchanged and wouldbe substantially zero.

[0086] As shown in FIG. 7, only a slight increase in the reflectancebrings about a shift of the reflectance profile toward the longwavelength side with the steep shape kept maintained. When it comes tothe reflectance with the light having a wavelength of 410 nm, which isused for reading information, the reflectance is increased by about 80%if the refractive index of the super resolution film 4 is increased toonly 1.8, though the reflectance is only about 3% in the case where therefractive index of the super resolution film 4 is 1.7. This impliesthat it is possible to set the reflectance in the optical mask portionat about 3% and to set the reflectance in the optical opening portion atabout 80%. In other words, it is possible to shield the information fromthe optical mask portion and to read selectively the information fromthe optical opening portion. The particular dependence of thereflectance on the light intensity well conforms with the dependency ofthe transmittance on the light intensity described previously inconjunction with FIG. 3B in the effect of diminishing the diameter ofthe light-beam, though there is a difference in wording between thetransmittance and the reflectance.

[0087] The shift of the reflectance profile described in conjunctionwith FIG. 7 also takes place in the case where the thin filminterference section 3 is formed of a single transparent thin film 3Aalone, as apparent from curve 41 shown in FIG. 6. In this case, however,the reflectance does not exhibit a steep change in accordance with thechange in the wavelength. Therefore, even if the refractive index of thesuper resolution film 4 is changed to some extent, the reflectance withthe light having a wavelength of 410 nm, which is used for readinginformation, is scarcely changed. In other words, it is impossible toobtain a sufficient super resolution effect.

[0088] In order to obtain a sufficient super resolution effect, it isnecessary for the thin film interference section 3 to be formed of atleast two transparent thin films 3A, 3B, and it is desirable to employ alaminate structure of at least three transparent thin films laminatedone upon the other such that the adjacent transparent thin films differfrom each other in the refractive index. In general, the upper limit inthe number of transparent thin films that are laminated one upon theother is determined such that the thickness of the laminated structuredoes not exceed the focal depth of the light-beam.

[0089] In order to obtain a large super resolution effect, it isnecessary to employ the laminate structure of the thin film interferencesection 3 and the super resolution film 4. In addition, it is desirableto determine appropriately the thickness, the refractive index, thestacking order, and the number of films stacked in the thin filminterference section 3 as well as the refractive index and the thicknessof the super resolution film 4 such that the reflectance assumes thelowest value when irradiated with any one of the light having a lowintensity and the light having a high intensity, as apparent from FIG.7. It should be noted, however, that, in order to obtain a larger superresolution effect, it is desirable for the super resolution film 4 tohave a large thickness within the focal depth of the light-beam. Suchbeing the situation, it is not desirable to limit the thickness of thesuper resolution film 4 in an attempt to achieve the reflectancedescribed above. In other words, it is desirable to increase the designflexibility.

[0090] In order to increase the design flexibility, it is effective toarrange the second thin film interference section referred to previouslybetween the super resolution film 4 and the reflective film 6. Thepresence of the second thin film interference section makes it possibleto realize the reflectance described above without limiting thethickness of the super resolution film 4.

EXAMPLE 2

[0091] The ROM disk 1 shown in FIG. 5 was prepared as in Example 1,except that the thin film interference section 3 of the ROM disk 1 wasprepared by alternately laminating three transparent thin films 3A andtwo transparent thin films 3B (HLHLH). In Example 2, the material andthickness of each of the thin films were set as follows so as to obtainthe highest super resolution effect in the case of reading informationby using light having a wavelength λ of 413 nm.

[0092] Specifically, a polycarbonate substrate was used as thetransparent substrate 2, and pits each having a length of 0.2 μm to 0.6μm were formed as recording marks on one main surface of the transparentsubstrate 2. Each of the transparent thin films 3A was formed of ZnShaving a refractive index of 2.4 and the thickness of which was set at68.3 nm so as to obtain an optical film thickness of λ/4. On the otherhand, each of the transparent thin film 3B was formed of SiO₂ having arefractive index of 1.5, and the thickness of which was set at 42.7 nmso as to obtain an optical film thickness of λ/4. The super resolutionfilm 4 was formed of a material having a refractive index n of 1.7 andan extinction coefficient k of approximately zero when irradiated withlight having a low intensity, the refractive index n of which beingchange to 1.8 and the attenuation coefficient k remaining substantiallyunchanged when irradiated with light having a high intensity. Also, thethickness of the super resolution film 4 was set at 74 nm so as tominimize the reflectance of the ROM disk 1 in the initial state in whichthe disk 1 is not irradiated with light. Further, the reflective film 6was formed of AlTi and the thickness of the reflective film 6 was set at100 nm.

[0093] For comparison, an additional ROM disk was prepared as above,except that the thin film interference section 3 was not included in theROM disk for the comparative case.

[0094] The dependence of the CNR (Carrier to Noise Ratio) value on thepit length was examined by using a reproduction evaluation machinehaving a Kr⁺ gas laser as a light source in respect of the ROM disk 1prepared in Example 2 and the ROM disk for the comparative case.Incidentally, the reproducing wavelength was set at 413 nm and thereproducing power was set at 1 mW in the reproduction evaluationmachine. FIG. 8 shows the results.

[0095]FIG. 8 is a graph showing the relationship between the pit lengthand the CNR value in respect of the ROM disk 1 prepared in Example 2 andthe ROM disk for the comparative case. In the graph of FIG. 8, the pitlength is plotted on the abscissa, with the CNR value being plotted onthe ordinate. Curve 56 shown in FIG. 8 represents the data obtained inrespect of the ROM disk 1 prepared in Example 2. Also, curve 57 shown inFIG. 8 represents the data obtained in respect of the ROM disk for thecomparative case.

[0096] As apparent from FIG. 8, when it comes to the ROM disk for thecomparative case, the CNR value is large where the pit length is notshorter than 0.4 μm. However, the CNR value is rapidly decreased wherethe pit length is less than 0.4 μm. The particular CNR profile isderived from the fact that, in the ROM disk for the comparative case, itis impossible to obtain a sufficient super resolution effect. On theother hand, the ROM disk 1 for Example 2 of the present inventionpermits maintaining a high CNR value even if the pit length is onlyabout 0.2 μm. The results clearly support that the thin filminterference section 3 is highly effective for improving thecharacteristics of the super resolution film 4.

EXAMPLE 3

[0097] The ROM disk 1 was prepared as in Example 2, except that a secondthin film interference section was formed between the super resolutionfilm 4 and the reflective film 6 and that the thickness of the superresolution film 4 was set at 300 nm. It should be noted that Example 3was intended to support that, if the second thin film interferencesection is arranged, it is possible to increase the thickness of thesuper resolution film 4, i.e., it is possible to increase the opticalpath, making it possible to obtain a higher super resolution effect.Incidentally, in Example 3, the second thin film interference film wasformed of a single layer of an AlN film having a refractive index of1.8. Also, the thickness of the second thin film interference sectionwas set at 100 nm in order to minimize the reflectance when irradiatedwith light having a low intensity.

[0098] The dependence of the CNR value on the pit length was alsoexamined for the ROM disk 1 as in Example 2. It was possible to obtain aCNR value and a super resolution effect higher than those obtained inExample 2.

EXAMPLE 4

[0099] In Examples 1 to 3 described above, the present invention isapplied to an optical recording medium of read-only type. On the otherhand, Example 4 is directed to a rewritable type optical recordingmedium.

[0100] Specifically, FIG. 9 is a cross sectional view schematicallyshowing the optical recording medium 1 according to Example 4 of thepresent invention. The optical recording medium 1 shown in FIG. 9 is aphase change type optical disk, and has a transparent substrate 2 madeof polycarbonate and provided with a spiral groove or concentric grooveson one main surface thereof. On the surface of the transparent substrate2 on which the groove(s) is formed, the first thin film interferencesection 3, the super resolution film 4, the second thin filminterference section 8, the recording film 5, a third thin filminterference section 9 and the reflective film 6 are stackedsuccessively. The first thin film interference section 3 is prepared byallowing a single transparent thin film 3B to be sandwiched between twotransparent thin films 3A. On the other hand, each of the second andthird thin film interference sections 8 and 9 is of a single layerstructure.

[0101] In Example 4, each of the transparent thin films 3A was formed ofTiO₂ having a refractive index of 2.5, and the transparent thin film 3Bwas formed of MgF₂ having a refractive index of 1.2. Also, the thicknessof each transparent thin film 3A was set at 40 nm and the thickness ofthe transparent thin film 3B was set at 85 nm so as to provide thequenching state when irradiated with light having a wavelength of 405nm. The super resolution film 4 was formed of a material having arefractive index n of 2.3 and an extinction coefficient k ofapproximately zero when irradiated with light having a low intensity,the refractive index n of which being increased to 2.4 and theattenuation coefficient k remaining substantially unchanged whenirradiated with light having a high intensity. Also, the thickness ofthe super resolution film 4 was set at 100 nm. The recording film 5 wasformed of Ge—Sb—Te, and each of the second and third thin filminterference sections 8 and 9 was formed of ZnS—SiO₂. Also, thereflective film 6 was formed of AgPdCu and the thickness of thereflective film 6 was set at 100 nm. Incidentally, in the optical disk1, the second and third thin film interference sections 8 and 9 serve toenhance the design flexibility and also serve to enhance the change inreflectance of the recording film 5. Also, these second and third thinfilm interference sections 8 and 9 act as protective films serving toprevent the light irradiated portion from being evaporated when therecording film 5 is irradiated with light.

[0102] For comparison, an additional optical disk of a phase change typewas prepared as above, except that the optical disk for the comparativecase did not include the thin film interference section 3.

[0103] The dependence of the CNR value referred previously on thedistance between adjacent recording marks was examined by using arecording-reproducing evaluation machine having a semiconductor laserwith a wavelength of 405 nm as a light source in respect of the phasechange type optical disk 1 prepared in Example 4 and the phase changetype optical disk for the comparative case. Incidentally, in recordinginformation on the optical disk 1, recording marks each having a lengthof 0.3 μm were formed in the recording film 5 with a single frequencywhile changing the distance between adjacent recording marks. During theinformation recording, the optical disk 1 was rotated at a linear speedof 6 m/s and the recording power was set at 9 mW. Also, the informationrecorded on the optical disk 1 was reproduced with a power of 1 mw whilerotating the optical disk at a linear speed of 6 m/s. FIG. 10 shows theresults.

[0104] Specifically, FIG. 10 is a graph showing the relationship betweenthe distance between adjacent recording marks and the CNR value inrespect of the phase change type optical disk 1 for Example 4 of thepresent invention and the phase change type optical disk for thecomparative case. In the graph of FIG. 10, the distance between adjacentrecording marks is plotted on the abscissa, with the CNR value beingplotted on the ordinate. Curve 58 shown in FIG. 10 represents the dataobtained in respect of the phase change type optical disk 1 for Example4 of the present invention. On the other hand, curve 59 shown in FIG. 10represent the data in respect of the phase change type optical disk forthe comparative case.

[0105] As apparent from FIG. 10, in the phase change type optical diskfor the comparative case, the CNR value is large in the case where thedistance between adjacent recording marks is not smaller than 0.3 μm.However, the CNR value is rapidly decreased in the case where thedistance between adjacent recording marks is smaller than 0.3 μm. Theparticular phenomenon is brought about by the fact that a stronginterference is generated between adjacent recording marks. Also, in thephase change type optical disk for the comparative case, there is alarge crosstalk from the adjacent track, with the result that the CNRvalue is not appreciably increased even where the distance betweenadjacent recording marks is large.

[0106] On the other hand, when it comes to the phase change type opticaldisk 1 for Example 4 of the present invention, a high CNR value ismaintained even if the distance between adjacent recording marks issmall, i.e., about 0.15 μm. Also, crosstalk is unlikely to take place inthe phase change type optical disk 1 for Example 4 of the presentinvention, making it possible to realize a CNR value markedly higherthan that of the phase change type optical disk for the comparativecase.

[0107] Incidentally, in Example 4 of the present invention, a phasechange type recording film was used as the recording film 5. However, itis also possible to use a magneto-optical recording film or a dyerecording film as the recording film 5. In this case, it is possible toobtain the effect similar to that described above.

EXAMPLE 5

[0108] The optical disk 1 shown in FIG. 5 were prepared, though thematerials of the super resolution film 4 were changed in severalfashions. In each of these optical disks 1, the super resolution film 4was formed of an optical material in which the refractive index n ischanged in accordance with the intensity of the irradiating light withthe extinction coefficient k, which is zero, left substantiallyunchanged. Then, the reflectance in the case of irradiation with lighthaving a low intensity and the reflectance in the case of irradiationwith light having a high intensity were measured in respect of each ofthese optical disks 1. FIG. 11 shows the result.

[0109] Specifically, FIG. 11 is a graph showing the effect given by thethin film interference section 3 to the reflectance of the optical disk1. In the graph of FIG. 11, plotted on the abscissa is the value ofΔn/n_(L), where Δn represents the difference between the refractiveindex n_(H) in the case where the super resolution film 4 is irradiatedwith light having a high intensity and the refractive index n_(L) in thecase where the super resolution film 4 is irradiated with light having alow intensity, and n_(L) represents the refractive index as noted above.On the other hand, plotted on the ordinate of the graph of FIG. 11 isthe value of ΔR/d, where ΔR represents the difference between thereflectance R_(L) in the case of irradiating the disk 1 with lighthaving a low intensity and the reflectance RH in the case of irradiatingthe disk 1 with light having a high intensity, and d represents thethickness (nm) of the super resolution film 4. Curves 61 to 65 are shownin the graph of FIG. 11. It should be noted that curves 61 to 64represent the data in the cases where materials exhibiting therefractive indices n_(L), when irradiated with light having a lowintensity, of 1.4, 1.7, 2.3 and 3.5, respectively, were used for formingthe super resolution film 4. On the other hand, curve 65 in FIG. 11represents the data on an optical disk equal in construction to theoptical disks of Example 5, except that the thin film interferencesection 3 was not formed in the optical disk and that the refractiveindex n_(L) of the super resolution film 4 was about 2.

[0110] As apparent from FIG. 11, it is possible to obtain ΔR/d that isat least two times as large as that in the case where the thin filminterference section 3 is not formed by setting the value of(ΔR/d)/(Δn/n_(L)) at 6.7×10⁻³ or more. To be more specific, it ispossible to obtain a sufficient super resolution effect even in the caseof using a material having a small value of An for forming the superresolution film 4 by constructing the optical disk 1 to meet therelationship represented by the inequality given below:

Δn/n _(L)≦150×(|R _(H) −R _(L))/d

[0111] Also, in the case of using a material having a large value of Δnfor forming the super resolution film 4, it is possible to obtain afurther excellent super resolution effect.

EXAMPLE 6

[0112] The optical disks 1 shown in FIG. 5 were prepared, though therefractive index of the transparent thin film 3B was set at 1.5 and therefractive index of the transparent thin film 3A was changed in severalfashions. The dependence of the reflectance in the case of irradiationwith light having a high intensity on the wavelength was examined inrespect of each of these optical disks 1. FIG. 12 shows a part of theresults.

[0113] Specifically, FIG. 12 is a graph showing the relationship betweenthe refractive index of the transparent thin film 3A and the reflectanceof the optical disk 1. The wavelength of the light irradiating theoptical disk is plotted on the abscissa of the graph, with thereflectance when irradiated with the light being plotted on theordinate. Curves 71 to 74, which are shown in the graph of FIG. 12,represent the data covering the cases where the refractive indices ofthe transparent thin films 3A were 2.3, 2.1, 1.9 and 1.7, respectively.As shown in FIG. 12, the reflectance profile is made steeper withincrease in the difference between the refractive index of thetransparent thin film 3A and the refractive index of the transparentthin film 3B.

[0114] Then, the difference AR between the reflectance R_(L) of the disk1 in the case of irradiation with light having a low intensity and thereflectance R_(H) of the disk 1 in the case of irradiation with lighthaving a high intensity was obtained. The wavelength of the irradiatinglight was 410 nm. FIG. 13 shows the results.

[0115] Specifically, FIG. 13 is a graph showing the relationship betweenthe difference ΔR in the reflectance of the optical disk 1 and thedifference An in the refractive index between the transparent thin film3A and the transparent thin film 3B. In the graph of FIG. 13, thedifference An in the refractive index is plotted on the abscissa, withthe difference AR in the reflectance being plotted on the ordinate. Asshown in FIG. 13, the difference AR in the reflectance is increasedprominently with increase in the difference Δn in the refractive indexin the case where the difference Δn in the refractive index falls withina range of between 0.2 and 0.6. However, the difference ΔR in thereflectance is substantially saturated if the difference Δn in therefractive index is increased to 0.6 or higher. It follows that it isdesirable for the difference Δn in the refractive index to be notsmaller than 0.2, more desirably, not smaller than 0.6.

EXAMPLE 7

[0116] In each of Examples 1 to 6, the super resolution film 4 is formedof a material in which the real part of the complex refractive index ischanged in accordance with the intensity of the irradiating light, withthe imaginary part of the complex refractive index left substantiallyunchanged. In Example 7, however, the super resolution film 4 is formedof a material in which the imaginary part of the complex refractiveindex is changed in accordance with the intensity of the irradiatinglight.

[0117] Specifically, in Example 7, the optical disk 1 shown in FIG. 5was formed except that, the thin film interference section 3 wasprepared by alternately stacking three transparent thin films 3A andthree transparent thin films 3B (LHLHLH). In Example 7, each of thetransparent thin films 3A was formed of SiO₂ and the thickness of eachfilm 3A was set at λ/4n or in the vicinity of λ/4n, where λ is 410 nmand n is 1.5. On the other hand, each of the transparent thin films 3Bwas formed of ZnS, and the thickness of each film 3B was set at λ/4n orin the vicinity of λ/4n, where λ is 410 nm and n is 2.4. Further, inExample 7, the super resolution film 4 was formed of a material having arefractive index n of 2.3 and an extinction coefficient k of 0 whenirradiated with light having a low intensity, the extinction coefficientk being increased to 0.5 and the refractive index n being left unchangedat 2.3 when irradiated with light having a high intensity. The superresolution film 4 is, for example, a semiconductor film or asemiconductor fine particle dispersion film. In the case of using such afilm, the super resolution film 4 exhibiting changes in the complexrefractive index as described above can be obtained by increasing theoptical concentration, compared with the system in which the refractiveindex n is changed in accordance with intensity of the irradiatinglight. Also, another material is used for forming the super resolutionfilm 4, it is possible to obtain the super resolution film 4 whichexhibits changes in the complex refractive index as noted above by, forexample, decreasing the amount of the solvent.

[0118]FIG. 14 is a graph showing the relationship between the intensityof the irradiating light and the reflectance in respect of the opticaldisk 1. In the graph of FIG. 14, the wavelength of the irradiating lightis plotted on the abscissa, with the reflectance of the optical disk 1being plotted on the ordinate. Curves 81 to 84 shown in the graph ofFIG. 14 represent the data covering the cases where the intensity of theirradiating light was controlled to permit the super resolution films 4irradiated with light having a wavelength of 410 nm to exhibit theextinction coefficients of 0, 0.1, 0.2 and 0.5, respectively.

[0119] Focusing attention on the reflectance in the case of irradiationwith light having a wavelength of about 410 nm, the reflectance is onlyabout 5% when irradiated with light having a low intensity, i.e., wherethe extinction coefficient k is zero. On the other hand, the reflectanceis increased to reach about 60% when irradiated with light having a highintensity, i.e., where the extinction coefficient k is 0.5. It followsthat it is possible to obtain a sufficient super resolution effect inthe optical disk 1 of Example 7.

[0120] Also, focusing attention on the reflectance in the case ofirradiation with light having a wavelength of about 420 nm, thereflectance obtained when irradiated with light having a low intensityis higher than the reflectance obtained when irradiated with lighthaving a high intensity. What should be noted is that the relationbetween reflectances in the case of irradiation with light having awavelength of about 420 nm is opposite to that in the case ofirradiation with light having a wavelength of about 410 nm. This clearlysupports that it is possible to obtain a super resolution effect bysetting appropriately the wavelength of the irradiating light even inthe case where the super resolution film 4 is formed of a material whoseextinction coefficient k is decreased by increasing the intensity of theirradiating light. Incidentally, it is possible to cause a shift of thereflectance profile shown in FIG. 14 toward the short wavelength side orlong wavelength side by controlling the laminate structure employed inthe optical disk 1 as well as the thickness and refractive index of eachof the various thin films, as described previously in conjunction withFIG. 7. It follows that, if such a control is performed, it is possibleto obtain a sufficient super resolution effect by using the light havinga suitable wavelength, which is used in general, even in the case wherethe super resolution film 4 is formed of an optical material whoseextinction coefficient k is decreased by increasing the intensity of theirradiating light.

[0121] As described above, the present invention makes it possible toobtain a sufficient super resolution effect in both causes where thesuper resolution film 4 is formed of an optical material whoserefractive index n or extinction coefficient k is increased byincreasing the intensity of the irradiating light and where the superresolution film 4 is formed of an optical material whose refractiveindex n or extinction coefficient k is decreased by increasing theintensity of the irradiating light. Similarly, a sufficient superresolution effect can be obtained in the case where the super resolutionfilm 4 is formed of an optical material in which both the refractiveindex n and the extinction coefficient k are changed by increasing theintensity of the irradiating light. In other words, the presentinvention makes it possible for an optical material whose complex indexof refraction is changed by increasing the intensity of the irradiatinglight to be used as the material for forming the super resolution film 4even if the amount of change is small.

[0122] As described above, a super resolution film and a thin filminterference section are used in combination in the present invention,making it possible to obtain a very high super resolution effect.Therefore, according to the present invention, it is possible to realizea recording density higher than that in the prior art. Also, where therecording density is made equal to that in the prior art, the presentinvention permits suppressing a read error. Further, the presentinvention makes it possible for the super resolution film to be formedof an optical material whose complex refractive index is changedslightly by increasing the intensity of the irradiating light.

[0123] To reiterate, the present invention provides an optical recordingmedium and a recording-reproducing apparatus capable of achieving a highrecording density. The present invention also provides an opticalrecording medium and a recording-reproducing apparatus that are unlikelyto give rise to a read error. Further, the present invention provides anoptical recording medium and a recording-reproducing apparatus that makeit possible to use a material whose optical constant is changed slightlyby the light irradiation for forming a super resolution film.

[0124] Additional advantages and modifications will readily occur tothose skilled in the art. Therefore, the invention in its broaderaspects is not limited to the specific details and representativeembodiments shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. An optical recording medium, comprising: asubstrate; a reflective film provided on said substrate and having arecessed portion or a projecting portion as a recording mark on asurface thereof; a super resolution film made of an optical materialwhose complex refractive index is changed in accordance with anintensity of a light irradiating said super resolution film; and a firstthin film interference section consisting of a plurality of transparentthin films, said plural transparent thin films being stacked oneanother, each two of said plural transparent thin films adjacent to eachother being different from each other in refractive index, and saidsuper resolution film and said first thin film interference sectionforming a laminate structure between said substrate and said reflectivefilm or on the reflective film.
 2. The optical recording mediumaccording to claim 1 , wherein said super resolution film is interposedbetween said first thin film interference section and said reflectivefilm.
 3. The optical recording medium according to claim 2 , furthercomprising a second thin film interference section consisting of atleast one transparent thin film and interposed between said superresolution film and said reflective film.
 4. The optical recordingmedium according to claim 1 , wherein a number of the transparent thinfilms constituting said first thin film interference section is at leastthree.
 5. The optical recording medium according to claim 1 , whereineach two of said plural transparent thin films adjacent to each otherare different from each other in refractive index by at least 0.2. 6.The optical recording medium according to claim 1 , wherein each of saidplural transparent thin films constituting said first thin filminterference section substantially satisfies a relationship representedby an equation given below: d=λ/4n+m×λ/n where d denotes a thickness ofone of said plural transparent thin films, n denotes a refractive indexof said transparent thin film, λ denotes a wavelength of the light, andm denotes 0 or natural number.
 7. The optical recording medium accordingto claim 1 , wherein real part n of the complex refractive index of saidoptical material is higher than imaginary part k of the complexrefractive index of the optical material in a rate of change relative tothe intensity of the light.
 8. The optical recording medium according toclaim 7 , wherein said medium satisfies a relationship denoted by aninequality given below: |n ₂ −n ₁ |/n ₁≦150×|R ₂ −R ₁ |/d where n₁denotes a real part of the complex refractive index of said opticalmaterial when irradiated with light having a first intensity, n₂ denotesa real part of the complex refractive index of said optical materialwhen irradiated with light having a second intensity higher than saidfirst intensity, d denotes a thickness (nm) of said super resolutionfilm, R₁ denotes a reflectance of said medium when the complexrefractive index of said optical material has the real part n₁, and R₂denotes a reflectance of said medium when the complex refractive indexof said optical material has the real part n₂.
 9. The optical recordingmedium according to claim 1 , wherein real part n of the complexrefractive index of said optical material is lower than imaginary part kof the complex refractive index of the optical material in a rate ofchange relative to the intensity of the light.
 10. An optical recordingmedium, comprising: a substrate; a reflective film provided on saidsubstrate; a super resolution film made of an optical material whosecomplex refractive index is changed in accordance with an intensity of alight irradiating said super resolution film; a first thin filminterference section consisting of a plurality of transparent thinfilms, said plural transparent thin films being stacked one another,each two of said plural transparent thin films adjacent to each otherbeing different from each other in refractive index, and said superresolution film and said first thin film interference section forming alaminate structure between said substrate and said reflective film or onthe reflective film; and a recording film provided between said laminatestructure and said reflective film.
 11. The optical recording mediumaccording to claim 10 , wherein said super resolution film is interposedbetween said first thin film interference section and said reflectivefilm.
 12. The optical recording medium according to claim 11 , furthercomprising a second thin film interference section consisting of atleast one transparent thin film and interposed between said superresolution film and said reflective film and a third thin filminterference section consisting of at least one transparent thin filmand interposed between said recording film and said reflective film. 13.The optical recording medium according to claim 10 , wherein a number ofthe transparent thin films constituting said first thin filminterference section is at least three.
 14. The optical recording mediumaccording to claim 10 , wherein each two of said plural transparent thinfilms adjacent to each other are different from each other in refractiveindex by at least 0.2.
 15. The optical recording medium according toclaim 10 , wherein each of said plural transparent thin filmsconstituting said first thin film interference section substantiallysatisfies a relationship represented by an equation given below:d=λ/4n+m×λ/n where d denotes a thickness of one of said pluraltransparent thin films, n denotes a refractive index of the transparentthin film, λ denotes a wavelength of the light, and m denotes 0 ornatural number.
 16. The optical recording medium according to claim 10 ,wherein real part n of the complex refractive index of said opticalmaterial is higher than imaginary part k of the complex refractive indexof the optical material in a rate of change relative to the intensity ofthe light.
 17. The optical recording medium according to claim 16 ,wherein said medium satisfies a relationship denoted by an inequalitygiven below: |n ₂ −n ₁ |/n ₁≦150×|R ₂ −R ₁ |/d where n₁ denotes a realpart of the complex refractive index of said optical material whenirradiated with light having a first intensity, n₂ denotes a real partof the complex refractive index of said optical material when irradiatedwith light having a second intensity higher than said first intensity, ddenotes a thickness (nm) of said super resolution film, R₁ denotes areflectance of said medium when the complex refractive index of saidoptical material has the real part n₁, and R₂ denotes a reflectance ofsaid medium when the complex refractive index of said optical materialhas the real part n₂.
 18. The optical recording medium according toclaim 10 , wherein real part n of the complex refractive index of saidoptical material is lower than imaginary part k of the complexrefractive index of the optical material in a rate of change relative tothe intensity of the light.
 19. A recording-reproducing apparatus,comprising: an optical recording medium comprising a substrate, areflective film provided on said substrate, a super resolution film madeof an optical material whose complex reflective index is changed inaccordance with an intensity of a light irradiating said superresolution film, a first thin film interference section consisting of aplurality of transparent thin films, said plural transparent thin filmsbeing stacked one another, each two of said plural transparent thinfilms adjacent to each other being different from each other inrefractive index, and said super resolution film and said first thinfilm interference section forming a laminate structure between saidsubstrate and said reflective film or on the reflective film, and arecording film provided between said laminate structure and saidreflective film; a recording mechanism configured to irradiate saidrecording film with a recording light so as to form recording markscorresponding to information to be recorded in the recording film; and areproducing mechanism configured to irradiate the recording film withlight and detect the light reflected from the optical recording mediumso as to reproduce the information recorded as said recording marks inthe recording film.
 20. The recording-reproducing apparatus according toclaim 19 , wherein each of said plural transparent thin filmsconstituting said first thin film interference section substantiallysatisfies a relationship represented by an equation given below:d=λ/4n+m×λ/n where d denotes a thickness of one of said transparent thinfilm, n denotes a refractive index of said transparent thin film, λdenotes a wavelength of the light, and m denotes 0 or natural number.